Magnetic core structure



Jn. 15, 1946. A D FQRBES v 2,393,038

MAGNETIC CORE STRUCTURE Filed July 15, 1942 CR A #an 0. Forbes. @Ju/l f M ATTORNEY Patented Jan. l5, 1946 MAGNETIC CORE STRUCTURE `Allan D. Forbes, Sharon, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application July l5, 1942, Serial No. 450,971

Claims.

My invention relates to magnetic core structures for use in electrical induction apparatus such as transformers.

In electrical induction apparatus, such a." distribution or power transformers, it has been the usual practice to employ a core structure of magnetic material formed of a stack of layers of thin sheets or laminations of magnetic material such, for example, as hot-rolled silicon steel. A portion of the magnetic structure is employed as a winding leg about which the copper circuit conducting windings of the transformer are wound. In order to keep the cost of the transformers as low as possible, it is desirable that the length of the mean turn of the current carrying conductor about the winding leg be as small as practicably consistent with permissible losses and power perfomance of the transformer.

The smaller the cross-section of the magnetic material the greater will be the ux density necessary to develop a given electromotive force and the greater will be the exciting current flowing in the primary winding. The permissible exciting current for a given size of Vtransformer has been a factor in limiting the permissible reduction in the cross-section oi' the magnetic material.

The exciting current is made up of two components or parts, namelyy the magnetizing current that is necessary to force the lines of magnetic flux through the iron circuit of the core, and the iron loss current necessary to supply losses of energy in the iron core structure, such as those due to hysteresis and eddy current losses. The component of magnetizing current is controlled very largely bythe permeability cf the material usedin the core structure, and the watts loss component of current is controlled by the iron loss characteristic of the material.

The magnetic iron or steel usually employed in transformer core structures is a high grade hotrolled silicon steel having a value of permeability and watts loss per cubic volume of substantially constant value over a considerable departure in the direction of the sheet along which the lines of magnetizing ilux pass from the direction of rolling. More expensive steels have been developed having much higher permeability and much lower losses in the direction of rolling. In such material the permeability varies considerably as the direction of the lines of magnetizing force vary from the direction of rolling of the material and is the highest in the direction of rolling. The watts loss per cubic volume or per cubic weight of the material likewise varies markedly as the direction of magnetizing ilux varies from the direction oi' rolling oi' the material and is lowest in the direction of rolling.

The permeability oi' the new steel in the direction oi' rolling is considerably higher at operating densities (say 13 to 15 kilogausses per square centimeter) than the permeability of commercial grades of hot-rolled silicon steel at the same densities. Likewise the watts loss per unit volume or unit weight at operating densities when magnetized in the direction of rolling is lower than for commercial grades of hot-rolled silicon steel at the same density.

It will be seen, therefore, that if a transformer core is formed of sheet steel having a preferred or greater permeability and a preferred or lesser watts loss per unit volume when the flow of flux is through the steel in the direction oi rolling than when it is through the material in other directions and the sheets are so arranged that the lines of magnetic ilux pass through the steel in the direction of rolling, a higher permeability and lower watts loss will result, permitting the use of higher ilux densities than are permissible with the usual grade of silicon steel, and a reduction of the cross-section of the magnetic material otherwise required.

Magnetic cores have been built with a high grade steel having a preferred permeability and low watts loss in the direction of rolling in which stacks of laminations have been used for the winding leg and yoke portions of the core. In such cases, it has been the practice to employ yoke portions of the core having a greater width from the window to the outside of the core than the width oi' the leg portions and arrangements of the sheets of steel forming the core have been worked out for eil'ecting eilicient joints. These arrangements, however, are not suitable to structures in which the yoke and leg portions have the same width from the window to the outside edge oi' the core structure.

It is an object oi' the invention to provide a core structure for an electrical induction apparatus made up of stacked sheets of steel having a preferred permeability or easiest magnetization in the direction of rolling so positioned in the structure that the lines of magnetization correspond substantially to the directionV of rolling of the material and in which the width of the sheets for the core yoke and leg portions of the core are the same.

It is a further object of my invention to provide a core structure of the above indicated character employing combination butt and lap joints adjacent the corners of' a rectangular core structure in which the meeting edges are cut at an angle to the direction of the sheets to effect a high permeability of the core structure as a whole and as low an iron loss as possible.

Other objects and advantages of the invention will be apparent from the following description of a preferred embodiment thereof, reference being had to the accompanying drawing, in which:

Figures l and 2 illustrate the manner in which magnetic iiux passes through core members of different designs;

Fig. 3 illustrates an arrangement of stacked sheets in a prior art core structure in which the width of the yoke members is greater than that of the width of lthe leg members;

Fig. 4 illustrates an arrangement of the stacked sheets of steel in a transformer core structure organized in accordance with the invention;

Figs. 5 and 6 illustrate parts of the core structure used in the assembly shown in Fig. 3; and

Figs. and 8 illustrate parts of the core structure used in the assembly illustrated in Fig. 4.

Fig. l illustrates a conventional arrangement of the laminations or sheet steel punchlngs commonly used in a core structure built from high grade hot-rolled silicon steel comprising in each layer of the core structure four punchings or sheets I, 2, l and l, arranged in the same plane, the punchings I and 3 being alike and the punchings 2 and I being alike. The several punchings are rectangular in shape, the punching I extending from I a to I a', the punching 2 extending from 2a to 2a', the punching l extending from 3a to la', and the punching l extending from 4a to la', so that one Vend of each oi the four punchings is adjacent the side of its adjoining punching. In this form oi construction, the position of the four punchings will be shifted in the adjacent layer to provide an overlap between the punchlngs in the adjacent layers so that the punching I will extend from Ibto I b', the punching 2 from 2b to 2b', the punching 3 from 3b to lb', and the punching l from Ib to Ib', the punchings in the second described layer forming the same general pattern as those in the ilrst described layer but with the positions of the adjoining edges shifted from the full line positions la, 2a, 3a and 4a to the dotted line positions Ib', 2b', 3b' and 4b'.

In the structure shown in Fig. 1, the core legs 2 and I may be winding legs of the core structure about which the primary and secondary windings of the transformer are placed and the connecting portions I and l form the yokes oonnecting the winding legs to complete the magnetic circuit of the core structure. In this structure the lines of the magnetic flux pass lengthwise throughl the straight portions of the legs and yoke, and flow about the corners of the structure something after the fashion indicated by the arrowed lines I, so that near the ends of the punchings the flux must pass sidewise of the members across the grain of the steel. This construction is entirely satisfactory for cores employing conventional hot-rolled steel where the permeability is not much diii'erent at right-angles to the direction of rolling from its value in the a,soa,oss

of the properties of the material, it is necessary to employ a construction in which the lines of flux follow the direction of rolling, or easiest magnetiution, at all points.

In Fig. 2 of the drawing it will be noted that the adjoining edges of the several sheets l, I and Il comprising a layer of the core material are cut on a diagonal or at an angle to the direction of the sheets. In this core the flux will tend to follow the direction of easiest magnetintion, as shown by the arrow lines II, although -this may not be the shortest path around the corner. By following this path of easiest magnetization or least resistance, the lines of ux avoid the necessity of passing crosswise to the direction of rolling near the ends of the sheets of each lamination.

In Fig. 3 a core structure is shown comprising stacks of sheet steel parts I4, Il, Il and I1 arranged about a window having a length L and a width W. The leg members Il and I1 are shown as having a width N and the yoke members Il and Il as having a greater width of N+1. The members I4 and Il are alike in shape and as best shown in Fig. 5, the inner edge of the sheet has a length W corresponding to the width of the core window. The sheets IB and Il are also alike and as shown in Fig. 6, have a length along the inner edge indicated by L+n. which is somewhat greater than the length L of the core window. The members are so arranged in the core structure that in each layer one end of the sheets comprising the leg members extends from a point Il at the inner angle of the window to a point Il at the other end of the inner edge of the sheet that is offset from the inner angle of the corner of the window. The diagonal ends of the sheets at one end extend from the inner angle of the window Il to a point 2i ofl'set from the outer corner of the 40 core and at the other end from the outer angle of the core 22 to the point Il which is onset from the inner angle of the window. In adjacent layers, the members I4 and II are reversed end for end and the members I8 and I1 are so positioned that the adjoining edges in one layer follow the full line outline Il, 2|, 22 and Il, and in the adjoining layer the dotted line outline Il', I l', 22' and 2i', providing an overlap between the sheets in the adjacent layers shown by the space between the lines I8-2I and It'-22'. The corresponding points in the two layers are given the same number but are primed in the dotted line DOsition.

This prior art core structure works well when the core and leg members of the structure are of diiferent widths, but, it will be appreciated that when the core and leg members are of the same width, the diagonal lines extending inwarddirection of rolling, but experience has shown that it is unsuited to the use of the new type steel having a much greater preferred permeability and lower watts loss when the lines of magnetic flux pass through the material in the direction of rolling than when these lines of flux pass through the material in other directions. Fior such new type steel. in order to make eiiicient use ly from the outer angle of the core and extending outwardly from the inner angle of the core would bring the adjoining edges of the sheets in adjacent layers opposite one another, eliminating the overlap and resulting in a core structure similar to that shown in Fig. 2, in which the superimposed layers are alike.

'Ihe principle illustrated with respect to Fig. 2 is made use of in the core structure shown in Fig. 4, the sheets of steel comprising the core being so arranged that the adjoining edges of the sheets in different layers extend on opposite sides of a line joining the inner comer of the window with the outer corner of the core to form an alternate butt and lap joint having a lower joint reluctance than would be the case ii the sheets in the several layers were superimposed upon each other.

In accordance with my invention, a core structure is provided in which the yoke and leg portions are of equal width N. The stacks of four punchings 24, 25, 26 and 2l are arranged about the core window, the sheets 24 and 26 corresponding to the sheet shown in Fig. 7, and the sheets 26 and 21 corresponding to the sheets shown in Fig. 8. The sheets 24 and 26 have a length along their inner edge of W+.r, which is somewhat greater than the width W of the core window and the sheets 25-21 have a length along their inner edge of L-i-, which is somewhat greater than the length L of the core window. In the full position of the sheets 24 and 26 comprising the yoke members in Fig. 4, it will be noted that the corner 3| of .the sheet is positioned at the inner angle of the window and the edge of the sheet extends to an angle point 32 which is in line with the extended inner edge along the length ofthe window, and from that point the adjoining edges between the sheets 24 and 25 and between sheets 26 and 21 extend along the lines 32-33 in a direction parallel to a line joining the inner angle of the window with the outer angle of the core; then along the outer edge of the core to the corner 34, and then along the edge length of the core to theI point 35, then parallel to the line adjoining the outer corner of the core with the inner angle of the window to the angle point 36, and from the angle point 36 to the corner of the window 3i along a line which includes the edge of the window and an extension thereof. In the adjoining layer, the members 24 and 26 will be reversed end for end so that the same outline will apply in reading the same numbers as primed, that is, the sequence 3i', 32', 33', 34', 35 and 36 outlining the corresponding points in the sheets. Likewise in adjacent layers, the members 25-21 are reversed end for end so that the edges of the leg members join with the edges of the yoke members.

It will be noted, therefore, that in each layer of sheets about the window each sheet 24, 25, 26 and 21 extends along its inner edge from a corner of the window at one end to an angle point in the sheet which is at a point displaced from the angle of the window at the other end by continuing the inner edge of the sheets a distance a: beyond the inner angle of the window. By reversing this pattern in adjacent layers so that the angle points 32 and 36 or 32' and 36 in adjacent layers are on opposite sides of a straight line joining the inner angle of the window with the outer angle of the core, an overlap between adjacent layers of constant width is provided. This overlap is the area shown between line 32 and 33 and line 36 and 35' or between line 32'-33' and 36-35.

It will be noted that in accordance with the structure shown in Fig. 4 comprising the invention, the benets of the diagonal cut as described with respect to Fig. 2 are provided without the necessity of providing widening of the yoke members, as is required in the structure shown in Fig. 3, In the structure shown in Fig. 4, the yoke members have the same width N as the leg members. This results in a saving in magnetic steel which in many applications is desirable.

It will also be noted that the overlap of the sheets adjacent the corners of the core may be varied by varying the dimension a: in the lengths W-l-:r and L+: of the sheets of steel shown in Figs. 7 and 8 respectively, without varying the width of the yoke member with respect to the leg member. This permits a more ilexible design of the joint structure than where the overlap is dependent upon the relative widths of the yoke and leg members.

It will also be noted that the sheets comprising the core structure 4 are identical in shape and differ only in length, so that an economy isw effected in that the same die construction may be used to make the same punchings and both punchings may be made from the same sheet of lsteel since the width required for both punchings is the same. 'I'he dies may be arranged to cut alternately a leg piece and then a yoke piece from the same strip oi steel since both pieces are of the same width and the same shape at the ends. Any variations in the gauge thickness of the sheets of material is thus minimized or eliminated in the assembled core. Where it is necessary to cut the yoke pieces from a different strip of material than the leg pieces and if the two strips should be of slightly different thicknesses, it is possible'that all of the yoke pieces may be thinner than the leg pieces, or vice versa, thus introducing diiliculties in assembling the core and in improperly distributing the losses between the core and yoke members. These difliculties are eliminated in the construction shown in Fig. 4.

It will be apparent to those skilled in the art that modifications in the detailed arrangements of the parts of the structure may be made within the spirit of my invention, and I do not wish to be limited otherwisev thanby the scope of the appended claims.

I claim as my invention:

l. In an electrical apparatus, a substantially rectangular magnetic core structure built up from layers of sheets of magnetic `material formed of steel having preferred greater permeability and lower watts loss in the direction of rolling than in other directions and so positioned in the structure that the direction of magnetization is substantially coincident with the direction of rolling, each layer comprising a plurality of sheets having adjoining edges adjacent the corners oi' the core structure and arranged about a substantially rectangular window, the sheets on all sides of the window being of the same width, the junctions of all adjoining edges at the outer corners of the magnetic core extending along lines formed by continuing the inner edge oi' one of the adjoining sheets at each corner of the window in a straight line in the direction of rolling slightly past the corner of the window to an angle point of the sheet and from that angle point in a continuous straight line at an angle oblique to the direction of rolling and parallel to a line connecting the corner of the windows with the corresponding outer corner of the core so that substantially all ilux in each sheet passes between junctions at opposite ends of each sheet in a continuous straight path along the direction of rolling, the line of junction between said adjoining edges of the sheets of magnetic material of one layer at any corner of the window extending outwardly from an angle point of the sheet ofl'set from the inner angle of the window on one side of and parallel to a line joining the inner corner of the window and the outer corner of the core, and the line of Junction between the adjoining edges of any adjacent layer at the same corner extending outwardlyfrom a point oiset from the inner corner of the window on the other side of and parallel to the line joining the inner corner of the window and the outer corner of the core, to provide an overlap` of substantially constant width between the adjacent junction lines of said adjacent layers.

2. In an electrical apparatus, a substantially rectangular magnetic core structure built up from layers of sheets of magnetic material formed of steel having preferred magnetic properties in the direction of rolling and so positioned in the structure that the direction of magnetization is substantially coincident with the direction of rollins. each layer comprising a plurality oi sheets having adjoining edges adjacent the corners of the core structure and arranged about a substantially rectangular window, the sheets on all sides of the window being of the same width, the junctions of all adjoining edges at the outer corners of the magnetic core extending along lines formed by continuing the inner edge of one of the adjoining sheets at each corner of the window in a straight line in the direction of rolling slightly past the corner of the window to an angle point in the sheet and from that angle point in a continuous straight line at angles oblique to the direction of rolling and parallel to a line connecting the corner of the window with the corresponding outer corner of the core so that substantially all ux in each sheet passes between junctions at opposite ends of each sheet in a continuous straight path along the direction of rolling, the line of junction between said adjoining edges of the sheets of one layer at a pair of alternate corners of the core extending from the inner angles of the window slightly past the corners of the window to the angle points of the sheets in opposite directions from the window in lines parallel to each other, the line of junction between the adjoining edges of said layer at the other pair of alternate corners of the core extending from the inner angles of the window slightly past the corner of the window to the angle points of the sheets in opposite directions from the window in lines parallel to each other and at right angles to the directions of the adjoining edges oi' the sheets in the same layer at the other two corners of the core, the sheets in any adjacent layer being so arranged that the directions in which the lines of junction extend from the corner of the window to the angle points of the sheets are the reverse oi' those of the first-named layer, the junction lines in the several adjacent layers from the angle points of the sheets to the outer edge of the core at any one corner of the core extending in parallel directions on the opposite sides of a line joining the corner of the window and the outer corner of the core to provide an overlap of substantially constant width between ,the junction lines of the adjacent layers.

3. In an electrical apparatus, a magnetic core structure built up from layers of sheets of masnetic material formed of steel having preferred magnetic properties in the direction of rolling and so positioned in the structure that the direction of magnetization is substantially coincident with the direction of rolling, each layer comprising a plurality of sheets arranged to form a substantially rectangular window, the sheets on all sides of the window being of the same width. the junctions of all adjoining edges o! any of the sheets in any layer extending along lines formed by continuing the inner edge ot one of the adjoining sheets at each corner of the window in a straight line in the direction oi' rolling slightly past the corner of the window to an angle point in the sheet and from that angle point in oontinuous straight lines at angles oblique to the direction of rolling and parallel to a line connecting the corner of the windows with the oorrespondlng outer corner oi the core so that substantially all flux in each sheet passes between junctions at opposite ends of each sheet in a continuous straight path along the direction of rolling, the lines of junction between the adjoining edges oi' the sheets comprising one layer being so arranged that the inner edge of each sheet outlining the window terminates at one end of the sheet at a corner oi' the window and extends slightly past a corner of the window at the other end to an angle point in the sheet, and from that angle point in a direction parallel to a line from the inner corner of the window to the outer corner of the core, the location of the lines o! junction between all adjoining edges being reversed in adjacent layers so that the angle P0ints in the adjacent layers at any comer are on opposite sides of and parallel to a line joining the corner of the window and the outer corner of the core to provide an overlap of substantially constant width between the adjacent lines of junction of the adjacent layers.

4. In an electrical apparatus, a magnetic core structure comprising leg member and yoke members at angles of substantially to each other to form a closed magnetic circuit about a rectangular window, the leg and yoke members bein! built up o! layers oi' sheet members formed oi steel having preferred magnetic properties in the direction of rolling and so positioned in the structure that the direction of magnetization is substantially coincident with the direction of rolling, each layer comprising a plurality oi sheets having adjoining edges between leg and yoke members adjacent the corners of the core, the sheets forming the leg and yoke members being of the same width, the sheets in the several layers having their inner and outer lineal edges in alignment and the junctions of all adjoining edges between the sheets at substantially 45 to the lineal edges of the members so that substantially all flux in each sheet passes between junctions at opposite ends of each sheet along a continuous straight path in the direction of rolling, the junctions of all the adjoining edges of said sheet members in a given layer being oilset from a line joining the corner of the window and the outer corner oi the core, the junctions of the adjoining edges of the sheet members in an adjacent layer at the corresponding corners of the core being on opposite sides of and parallel to the line Joining the corner of the window and the corner of the core and parallel thereto to provide an overlap of substantially constant width between the adjacent junction lines of the adjacent layers, the yoke members having the ends adjoining leg members in the same layer offset differently at the opposite ends thereof with respect to the comers of the structure.

5. A magnetic core comprising a plurality oi layers of assembled lamlnatlons formed lengthwise from strip material having the most favorable magnetic direction lengthwise o! the strip, adjacent laminations of layers having substantially mitered butt joints between ends so that a minimum of tlux passing from one lamination to another will pass crosswise of the most favorable magnetic direction, said mitered butt joints being displaced from an adjacent diagonal running from the inside corners to the outside corners oi the assembled laminations at at least some corners oi' the assembled laminations, adjacent joints of contiguous layers at said corners being substantially symmetrically displaced on opposite sides of the adjacent diagonal with a major portion of said joints being parallel to the adjacent diagonals.

6. A magnetic core comprising a plurality of layers of assembled laminations formed lengthwise from strip material having the most favorable magnetic direction lengthwise of the strip, adjacent laminations of each layer having substantially mitered butt joints between ends so that a minimum of flux passing from one lamination to another will flow crosswise of the most favorable magnetic direction, said mitered butt joints of each layer being broken line joints with a major portion of said broken line joint surface being substantially parallel with an adjacent diagonal running from the inside corner to the outside corner of the assembled laminations.

'1. A magnetic core comprising a plurality of layers of assembled laminations formed lengthwise from strip material having the most favorable magnetic direction lengthwise of the strip, adjacent laminations of each layer having substantially mitered butt joints between ends so that a minimum of flux passing from one lamination to'another will pass crosswise of the most favorable magnetic direction, each of said layers of laminations having broken line joints. a rst portion of said broken line joint of one layer being substantially parallel with the longitudinal axis of the assembled core and a corresponding first portion of the adjacent joint of a contiguous layer being substantially parallel with a lateral axis of the assembled core so as to provide overlapping of the adjacent laminations of contiguous layers.

8. A magnetic core comprising a plurality of layers of assembled laminations formed lengthwise from strip material having the most favorable magnetic direction lengthwise of the strip, said layers including at least four laminations assembled with diagonal butt joints at adjacent ends, at least two of said laminations of said layers having edge surfaces in six planes, edge surfaces in four of said planes forming the edges for said butt joints.

9. A magnetic core structure having core members extending at substantially right angles to each other forming an L-shaped corner, said core members each comprising a plurality of sheets of magnetic material having preferred magnetic properties in the direction of the sheet, the sheets in the adjoining core members adjacent the corner of the core being of the same width and arranged inv a like number of layers, the sheets in each of the two adjoining core members of any layer having their adjoining edges in alignment, the junctions between adjacent laminations of each layer of the magnetic material having ad-` joining edges between their ends extending in a straight line from a point oilset from the outer corner of the assembled sheets inwardly in a directiony parallel to a line connecting the outer corner of the core structure to the inner corner of the core structure for substantially the entire width of the sheet so that a minimum of flux passing from one lamination to another will pass crosswise of the most favorable magnetic direction, the line of junction between the adjoining edges of the sheets of magnetic material in one layer at the corner of the core .extending inwardly from a point offset from the outer corner of the core structure on the other side of the line joining the outer corner of the core structure than does the line of junction between the adjoining edges of the sheets in the adjacent layers.

l0. A magnetic core structure having core members forming an L-shaped corner, said core members each comprising a plurality of sheets of magnetic material, the sheets in the adjoining core members adjacent the corner of the core being of the same width and arranged in a like number of layers, the sheets in each of the two adjoining core members of any layer having their adjoining edges in alignment and extending in a straight line from a point oiset from the outer corner of the vassembled sheets inwardhr in a direction parallel to a line connecting the outer corner of the core structure to the inner corner of the core structure for substantially the entire width of the sheet so that a minimum of flux passing from one lamination to another will pass crosswise of the sheet, the line of junction between the adjoining edges of the sheets of magnetic material in one layer at the corner of the core extending inwardly from a point offset from the outer corner of the core structure on the other side of the line joining the outer corner of the core structure than does the line of junction between the adjoining edges of the sheets in the adjacent layers.

ALLAN D. FORBES. 

